The invention relates to a drive system for motor vehicles.
In the very great majority of motor vehicles, an internal combustion engine is provided in their drive system as the main drive; this engine is coupled to one or more driving axles in the main drive train via a driving clutch and a transmission. Secondary assemblies, such as pumps, compressors, air conditioners, and electric consumers, such as power controls, are increasingly used in motor vehicles; they are driven via a secondary drive train that is coupled with the main drive train of the motor vehicle.
It is also known, for reducing environmental pollution, to use hybrid drives in motor vehicles; then the drive system has both an internal combustion engine and an electric machine for selectively driving the motor vehicle. Once again, the internal combustion engine and the electric machine are located in the main drive train of the motor vehicle, and only the internal combustion engine is used to drive the secondary assemblies.
From German Patent Disclosure DE 197 50 497 A1, it is also known to operate the secondary assemblies in the motor vehicle with the main drive of the motor vehicle via an intermediate gear; the secondary assemblies also have an electric machine as a starter generator, which on the one hand for starting the engine operates as an electric motor in the main drive train and on the other, for supplying the on-board vehicle electrical system, operates in the generator mode for charging an accumulator battery.
Such vehicle concepts, optimized especially with regard to fuel consumption, require among other things that the engine in the main drive train be shut off when the vehicle is stopped at traffic lights; that is, they require a start and stop mode and a shutoff or disengagement of the engine in overrunning phases, that is, in the inertia-utility mode. Compared with conventional vehicles, this means approximately ten times as many starting cycles, that is, from 400,000 to 600,000 starting cycles. The additional starting cycles are essentially repeat starts, that is, starts with the engine at operating temperature. For these repeat starts, especially fast, quiet starting is demanded. On the other hand, in these motor vehicles, for reasons of comfort, up to five times more generator power is needed compared with conventional vehicles, especially since electrically triggered and operated secondary assemblies function more favorably in terms of consumption. In this case, generator power levels of more than 5 kW are desired. In addition, this power is sometimes needed even in engine idling, that is, for crankshaft speeds of less than 700 rpm, which cannot be achieved with the claw pole generators that are conventional today.
To make it possible for the starting and power demands in the generator mode to be met with a so-called starter generator, it is known from German Patent Disclosure DE 197 05 610 A1, to employ so-called impulse starting to reduce the starting power, and to continue to use a conventional, additional starter for extremely low starting temperatures.
In terms of the charge balance, however, concepts that use a starter generator to start the engine in the main drive train both in cold starting and in the start and stop mode are extremely critical, especially, since when the electrical system power is high, it is impossible to predict what driving and consumption cycles will have to be handled. The case where there is a demand on an air conditioner while the vehicle is stopped is even less favorable, since the conventional air conditioner compressor would have to be operated from the accumulator battery of the motor vehicle. This requires a cost-intensive design with high capacity and performance, which in turn tends to increase fuel consumption.
It has also already been proposed, in the earlier German Patent Application DE 198 529 41.4 that the secondary assemblies be driven by an additional secondary assembly drive, embodied as a smaller internal combustion engine; these secondary assemblies also include a starter generator, which is used in the generator mode on the one hand to supply the vehicle electrical system and on the other in the motor mode to start the secondary assembly drive. The main drive is started from the secondary assembly drive via a clutch and is then decoupled from the secondary assembly drive again.